The invention is based on a security system for a vehicle of the generic type having an anti-theft controller which conducts a data exchange with at least one function controller, particularly an engine controller, in order to control its operational readiness and having a data line which connects at least the anti-theft controller and the function controller and through which the data exchange is carried out. A known system of this type (WO 93/05 987) consists of a starting device with a code carrier and a device for reading the code carrier, a data bus connected to the reading device as well as of a plurality of units connected to the data bus for the control of vehicle functions. In order to ready the latter for operation, the reading unit sends them a code signal which it compares to a previously stored reference code. If at least for one function controller the code received by the reading unit and the previously stored reference code do not agree, the function controller sends an error signal via the data bus, with the signal leading to the immobilization of the vehicle, depending on the importance of the respective unit for the vehicle functions. If a thief wanted to bypass this known system by means of an own reading unit and own function controllers, he would have to replace all of the existing units. Because of the high expenditure connected to this, the known system offers a high degree of protection against vehicle theft. The implementation of the described system, however, departed from a vehicle bus that includes all electronic components. But especially vehicles in operation today do not yet have such a bus. The usefulness of such a bus for diagnosing the connected units is also limited because the evaluation of all diagnostic messages must always presuppose that the bus itself is working properly. A diagnosis of the state of the bus itself is not provided for.